Go for an electrically isolated one. Anything else is just dangerous.
EVERY inverter sold by Jaycar is electrically isolated. That's the Powertech brand - and Rays Outdoors now sells them as well.
Size the inverter to give some capacity above your requirements. If you want to run a 200W device, get a 300 or 400W inverter. If you're thinking of running a microwave, be aware that they suck about 1000W equating to over 80A drawn off your battery. You can get an inverter big enough, but that sort of load requires really heavy cabling and a battery that can stand an 80A constant load - generally only starter batteries (which are bad for deep cycle applications) and AGM batteries can withstand this sort of punishment. Gel and flooded cell deep cycle batteries can't deliver that much current without damage.
Don't fret too much about modified sine wave/pure sine wave. Modifieds will create some radio noise on AM but most things shouldn't suffer too much from their use. Pure sine wave is better, more expensive and designed to operate computers - but since laptops generally run a switchmode power supply, they're generally okay on modifieds.